In this proposal we describe experiments to define the specific functions of eosinophils and to determine the mechanisms by which eosinophils carry out these functions. In prior studies we have produced an antiserum to eosinophils (AES) which effectively ablates these cells from the body of the guinea pig and we have also described a protein, the major basic protein (MBP), which accounts for the bulk of the material in the guinea pig eosinophil granule. In the proposed experiments we will test a variety of models in which eosinophils are prominent to determine whether or not ablation of the eosinophil alters the functioning of the model. These experiments include further tests of the eosinophil on susceptibility and resistance to parasites, on experimental asthma in the guinea pig, on antibody formation, on fertility and on tumors. We have also shown that administration of AES increases the susceptiblity and reduces the resistance of guinea pigs to infestation with Trichostrongylus colubriformis and in the proposed experiments we will analyze the mechanisms by which the eosinophil exerts its influence on the parasite. Finally we desire to pursue the question of this role in the eosinophil in hypersensitivity reactions particularly on histamine release in the guinea pig and also determine whether or not a syndrome resembling the hypereosinophilic syndrome seen in humans can be produced in the rat and if so, what the role of the eosinophil is in this syndrome.